The present invention pertains to actuators and, more particularly, to an actuator driven by a pneumatic stepper motor, especially one suited to drive a member such as a paper machine headbox slice opening control spindle.
As illustrated in FIG. 1, a typical paper machine headbox 1 distributes pulp slurry or stock through a long horizontal slit opening 2 onto a traveling perforated web or "wire" 3. Transverse the direction of wire travel, paper density or "weight" can be changed by opening or closing a long stainless steel bar or "slice lip" 4 which comprises the top of the slit opening and, therefore, determines its height. Attached to the slice lip 4 are spindles or "slice rods" 5 which are typically spaced about 3 to 6 inches apart. By exerting a linear force through these spindles, "weight actuators" 6 can elastically deform the slice lip 4 into a shape which produces a slit opening that yields a paper sheet having a preselected weight that is uniform across the sheet.
Historically, the weight actuators driving the spindles were manually controlled. In recent years, however, automatic weight actuators have become available which operate in response to signals generated by weight, moisture and thickness sensors to precisely control distribution of pulp stock onto the wire.
To be effective, an automatic weight actuator must deliver a force ranging from 500 to about 4,000 pounds. At the same time, it must be very small so that it can fit into and onto a multitude of headbox styles which have many and varying nearby encumbrances.
Shown in FIG. 2 is an AccuRay model 4505 linear stepper slice actuator, manufactured by ABB Process Automation of Columbus, Ohio, which utilizes a pneumatic stepper motor, generally 7, driving through a harmonic gear reduction, generally 9, to a keyed power screw 11 which outputs to the slice rod 5. Air enters the actuator from a central air supply and is distributed to four individual solenoid valves 13 mounted on a stator 15. Through sequential operation of these valves, air flows into pressure diaphragms 17 located within an inner elastic flex gear 19. The diaphragms 17 form a circle around the stator 15 and are sealed by a one-piece, rubber membrane 21. Each solenoid valve 13 channels air to a pair of diaphragms 17 located opposite each other. Successive, overlapping pressurization of the diaphragm pairs produces a rotating elliptical deformation of the stationary elastic gear 19. This action results in a continuous meshing of teeth between the inner elastic gear 19 and an outer gear ring 23.
Since the elastic gear typically has 160 teeth while the outer gear typically has 162 teeth, one revolution of 16 successive, overlapping pressurizations causes a relative motion of two teeth between the gears. A very high level of precision is therefore achieved by using (162.div.2).times.16=1,296 steps to generate one rotation of the outer gear. Rotation of the outer gear is converted to linear motion through a power screw drive. Each rotation of the stepper motor typically produces 100 mils (2.5 mm) of linear slice rod travel. Each step, thus generates 0.100 in..div.1296=0.08 mils (2 microns) of slice lip movement.
Force output of pneumatic weight actuators using such harmonic gears is limited by the efficiency of the gear set which, in turn, is inversely related to friction created between the alloy steel flex gear and the iron rigid gear typically used in existing actuators.
As previously mentioned, an important dimension for harmonic weight actuators is overall diameter. The largest diameter driving element is the harmonic rigid gear. Since this gear size is determined by the torque rating of the drive train, a chief design problem is trying to minimize the diameter of the actuator housing were it is packaged around the outside diameter of the rigid gear.
In the present invention, polymeric materials are employed in the construction of the harmonic gear set. Preferably, the flex gear is formed of nylon and the rigid gear is formed of acetal. These materials offer a reduced co-efficient of friction between the gears as compared to the conventional gear materials. As harmonic gears require sliding contact between gear teeth, this lower co-efficient of friction results in a more efficient conversion of pneumatic energy into rotary motion. Further, the polymeric flex gear is more compliant than a similar metallic one and, therefore, offers less resistance to the pneumatic action of the pressurized membrane.
Two problems arise, however, when polymers are utilized as harmonic gear materials in high force, small diameter pneumatic actuators. The first problem is gear size. Typical applications require actuators capable of exerting 4,000 pounds of force. Existing polymeric gear sets adapted to produce such a force have an outside diameter of 3.4 inches. On new headboxes, however, actuators are commonly spaced only 3 inches apart.
The second problem is air entrapment between the membrane and the stator. As illustrated in FIG. 7, as the membrane pressurizes and depressurizes, small amounts of air become trapped in the stator grooves 31, under the membrane 21. A polymeric flex gear, providing less spring force than a metal gear in returning the membrane to its depressurized shape, allows more air to remain trapped under the sealing ribs 33. Thus a polymeric gear, used in conjunction with a conventional stator, has less engagement movement of flex gear teeth between pressurized and depressurized states. The (effects are 1) less work done by each pressurization, as the gear force is exerted over a shorter distance; 2) increased rubbing between gears when manually actuated in a depressurized state (manual actuation is required in periods of lost air pressure or electric power); 3) creep in the flex gear diameter due to trapped air which creates a residual expansion force on the flex gear during periods of non-actuation, which is typically over 95% of actuator life; and 4) increased wear of the membrane as it is moved inside the stator grooves by the trapped air.
It is, therefore, a primary object of the present invention to provide a pneumatic stepper motor weight actuator which captures the advantages of polymeric harmonic gears while occupying less space and eliminating the problems associated with air entrapment in the stator grooves.
Elimination of trapped air is accomplished by creating orificed air passages from the bottom of the longitudinal stator grooves to the center stator hole (which contains the manual adjustment shaft). By testing various size orifices and various numbers of air passages it was found that a hole about 0.010 in. diameter could vent trapped air to an ambient pressure region in the center stator hole without orifice blockage by lubricants or membrane wear particles during actuator life, and without any increase in actuator air consumption.
Advantageously, the actuator includes a rigid gear comprising a toothed polymeric sleeve adhesively bonded inside a metal cup. The cup provides the desired hoop strength while permitting a substantial reduction in gear diameter. Specifically, such a composite gear, capable of producing the desired 4,000 pounds of force, has an outside diameter of less than 3 inches, while an equivalent conventional polymeric gear has a diameter of about 4 inches.